Process for the preparation of coating compounds, aqueous dispersions of PU reactive systems and their use for coating.
1. Field of the Invention
This invention relates to a process for the preparation of finely dispersed, readily spreadable polyurethane reactive coating compounds having a viscosity of from about 3,000 to 22,000 mPa.s/25.degree. C. and a total solids content of from about 20 to 69% by weight, characterized by the dispersion in water of from about 20 to 69% by weight of isocyanate prepolymers preferably blocked by ketoximes in the presence of from about 0.05 to 5% by weight of emulsifiers, thickening of the thin dispersion by about 0.1 to 2.5% by weight of water soluble thickener preferably based on relatively high molecular weight polyethylene oxides and salts of poly(meth)acrylic acid or salts of carboxy methyl celluloses, optionally with the addition of other aqueous polymer solutions, polymer latices and/or polymer dispersions, optionally with the addition of diluents and the usual additives such as pigments, fillers, and stabilizers and the addition of approximately equivalent quantities (based on the isocyanate prepolymers) of polyamines, preferably (cyclo) aliphatic diamines, addition of the thickener and of the polyamines being carried out before, during or after dispersion while the aqueous polymer solutions, polymer latices and/or polymer dispersions are added after dispersion of the isocyanate polymers and before or after addition of the thickeners.
The present invention also relates to coating compounds prepared in accordance with the process and their use for the reactive coating of textiles or leather substrates.
2. Description of the Prior Art
The manufacture of artificial leather and coated textile sheet products based on polyurethanes, which are versatile in their application, forms part of the state of the art. Coating materials for fashion garments, work garments and rainwear, shoe uppers, luggage and upholstery material, tenting material, maps, conveyor belts, etc. are manufactured by the application of one or more layers by the transfer or direct coating process.
The polyurethanes are worked up as solutions of the completely reacted polyurethanes in organic solvents or in the form of their aqueous dispersions. Coatings having a solid content per layer of 10 to 50 g/m.sup.2, in particular 20 to 40 g/m.sup.2 may be produced from the solutions or dispersions, which are usually at concentrations of 20 to 50%, by the knife coating or the reverse roll coating process. Where solutions are used, recovery or ecologically harmless destruction of the solvents entails considerable expense.
Solvent-free systems such as aqueous polyurethane dispersions and particularly the low solvent or solvent-free high solid reactive systems which have recently been introduced into coating technology are therefore gaining increasingly in importance.
Whereas 30 to 50% polyurethane dispersions can be used to produce the conventionally thin layers containing 20 to 40 g/m.sup.2 per coat, the high solid reactive polyurethane systems described in German Offenlegungschriften No. 2,902,090 (U.S. Pat. No. 4,248,756) and No. 2,814,079 (U.S. Pat. No. 4,299,868) and in our own unpublished patent application No. P 3,120,596 (U.S. application, Ser. No. 378,152, filed May 14, 1982, now U.S. Pat. No. 4,446,293) which can be heat hardened with amine chain lengthening agents or cross-linking agents, are particularly suitable for the production of thick layers, for example for adhesive coats, with solid contents, for example, of 60 to 300 g/m.sup.2 per coat, but they are very limited in their application for thinner coats, for example containing 30 to 50 g/m.sup.2. The state of the art has been documented particularly in German Offenlegungsschrift No. 2,814,079. Their preparation starts with blocked isocyanate prepolymers or with blocked isocyanate prepolymer solutions containing very small quantities (e.g. up to 10%) of organic solvents. Limited quantities (3 to 50% by weight) of aqueous polymer dispersions or polymer solutions having a solids content of 20 to 70% by weight are then stirred in, and the diamines or polyamines are added. Relatively coarse, viscous dispersions or mixtures are obtained, which are preferably only used for relatively thick coatings of the order of 60 to 300 g/m.sup.2 and they are therefore preferably used for relatively thick adhesive coats.
The blocked isocyanate prepolymers in most cases have high viscosities, e.g. .gtoreq.80,000 mPa.s/20.degree. C. Addition of the dispersions or aqueous polymer solutions causes a relatively slight change in viscosity. The isocyanate prepolymers are miscible with aqueous dispersions or polymer solutions up to proportions of about 1:1.
When attempts are made to prepare lower viscosity mixtures, the mixtures of blocked isocyanate prepolymers with larger quantities of PU dispersion are unstable. Thin coatings on this basis, containing, for example, 40 g/m.sup.2, do not level out smoothly and have surface defects or cavities, especially when the transfer process is employed. They are therefore hardly suitable for top coats which must meet exceptionally high standards of appearance and quality.
It was therefore an object of the present invention to provide a process by which spread coating pastes of suitable viscosity could be obtained in the form of finely divided dispersions from heat hardening reactive polyurethane preparations based on blocked isocyanate prepolymers and polyamines for coating textile sheet products or leather by the transfer or direct coating method to form layers with low solid contents of only 10 to 50 g/m.sup.2 which would form flawless coatings of the same high quality as polyurethanes obtained from reactive polyurethane preparations, even when used as top coatings.